Microwave-Assisted Rapid Preparation of Mesoporous Phenolic Resin

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Energy, Environmental, and Catalysis Applications

Microwave-Assisted Rapid Preparation of Mesoporous Phenolic Resin Nanosphere towards Highly Efficient Solid Acid Catalyst Zhan Mao, Linqing Cao, Fei Zhang, and Fang Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10410 • Publication Date (Web): 07 Aug 2018 Downloaded from http://pubs.acs.org on August 8, 2018

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ACS Applied Materials & Interfaces

Microwave-Assisted Rapid Preparation of Mesoporous Phenolic Resin Nanosphere towards Highly Efficient Solid Acid Catalyst Zhan Mao,‡ Linqing Cao,‡ Fei Zhang and Fang Zhang* The Education Ministry Key Lab of Resource Chemistry and Shanghai Key Laboratory of Rare Earth Functional Materials, Shanghai Normal University, Shanghai 200234, China

‡

These authors contributed equally to this work.

*

(F. Z.) Email: [email protected]; Telephone: +86-21-64321673.

Keywords. Microwave-assisted polymerization; Mesoporous phenolic resin; Solid acid; Water-medium organic synthesis

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ABSTRACT. A novel microwave-assisted polymerization and self-assembly protocol was developed to prepare ordered mesoporous phenolic resin with nanospherical morphology (MPRN) for the first time. This unique strategy dramatically saved the synthesis time about two days with an energy-efficient way. Owing to its abundant phenyl groups in the framework, it was easily transformed to benzenesulfonic acid functionalized mesoporous phenolic resin (SO3H-MPRN) by simple sulfonation treatment. The obtained SO3H-MPRN sample still possessed large surface area, two-dimensional hexagonal mesoporous structure and uniform spherical shape. Importantly, due to its intrinsic organic framework, the pore surface of SO3H-MPRN was hydrophobic. Accordingly, it exhibited the excellent catalytic activity and selectivity in aqueous formaldehyde participated Prins reaction and water-medium Fisher-Indole reaction. On basis of material characterizations and the control experiments, this remarkable catalytic performance could be ascribed to the synergetic effect derived from its short mesoporous channel and hydrophobic pore surface, which resulted in the decreased reactant diffusion limitation and the reduced water competitive adsorption. Also, it was stable in water due to the periodically arranged acid species in the resin framework and thus was easily recycled and used repetitively for at least five times.


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1. Introduction Liquid acid catalysts are widely used for the large-scale petrochemical processes, the synthesis of high value fine chemicals and the utilization of biomass.1 Due to their corrosivity, toxicity and high cost of regeneration, the replacement of liqiud acids with solid acids attracts much attention for the development of environmentally friendly chemical transformations.2 To date, a variety of solid acids including zeolites, metal oxides, metal phosphates and acidic resins has shown good catalytic efficiency and the improved reusability in a wide range of acid-catalyzed reactions.3 However, most of them usually require an anhydrous reaction condition, which necessitates volatile, flammable and toxic organic solvents.4 The major reason is due to that the traditional solid acids are hydrophilic. Thus, water can easily adsorb on the surface of solid acids and then causes the deactivation of active species or the destruction of the support frameworks. These effects bring the reduced catalytic activity and/or stability of these catalysts in the chemical synthesis with water as the solvent, component or byproduct.5 On the contrary, enzymes perform as the extremely effective catalysts to promote many biological processes in aqueous medium. Their remarkable performances result from the precise pre-organization of the local environment and functional groups around the catalytic active sites.6 One of their distinct properties is that enzymes have the hydrophobic interiors and the hydrophilic exteriors, which isolate the active sites from bulk solvent (water).7 Accordingly, inspired by Nature, the design of solid acids with suitable hydrophobicity for chemical reaction in water receives increased attention. The obvious advantages of this system are that it could



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simultaneously reduce the liquid acids and organic solvents pollutions as well as the ease recovery and recycling of acid catalysts.8 Until now, different hydrophobic solid acids have been successfully prepared and they exhibited the enhanced catalytic performances in the various reactions such as hydration, condensation, esterification, acylation and cellulose depolymerization.9 Generally, a post-grafting strategy is used to fabricate hydrophobic solid acids by introducing the hydrophobic functional groups or polymers on the surface or the framework of the supports such as zeolites, silica and metal oxide.10 However, this protocol usually causes the reduction of the surface area and pore size of the catalysts, leading to the increased diffusion resistance. Therefore, the direct use of hydrophobic support for the generation of hydrophobic solid acids is believed as a promising way to address this drawback.11 In this context, porous polymers have the obvious advantages such as controllable hydrophobicity, flexible framework, excellent stability and easy functionalization.12 Nevertheless, the traditional porous polymers often exhibit the relatively small pore size (< 2.0 nm), which unavoidably reduces the accessibility of the active sites, particularly in the presence of large size reactants.13 Recently,

Zhao

group

reported

the

preparation

of

ordered

mesoporous

phenol-formaldehyde resin by the hydrogen-bond-driven self-assembly of resol oligomers with block copolymers.14 The obtained mesoporous resin possesses excellent porosity properties with high surface areas (600-650 m2/g), large pore volumes (0.6-0.7 cm3/g) and uniform mesopores (3.0-5.0 nm). All these features are favorable for the catalytic applications.15 But, the synthetic process of this kind of


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resin is time-consuming up to several days since it needs five steps including resol oligomers

preparation,

thermal

induced

cross-linking,

self-assembly-driven

mesostructure formation and template removal, which seriously restricts its practical use as the catalyst support. Therefore, the development of a time and energy efficient protocol for the preparation of ordered mesoporous resin is of great significance for the future application. The use of microwave irradiation in polymer preparation has recently developed into a powerful tool with distinct properties.16 It has precise temperature and pressure control, which can improve the safety and reproducibility. Also, it is homogeneous, rapid, and volumetric heating, resulting in short reaction time, low energy consumption and controllable polymerization as well as the decreased thermal decomposition and/or side reactions.17 As expected, this strategy has been explored in the

step-growth

polymerization,

ring-opening

polymerization

and

radical

polymerization. A series of polymers such as polyamides, poly(urea)s, polyesters and polystyrene was conveniently obtained in this eco-friendly way.18 However, to our best knowledge, the utilization of direct microwave-assisted polymerization for the preparation of ordered mesoporous polymer has not been investigated yet.19-21 Herein, we reported that the first synthesis of ordered mesoporous phenolic resin nanosphere by microwave heating the resol oligomers/block copolymer composite solution. This one stone with two birds synthetic protocol efficiently saves about two days by creatively changing two most time-consuming steps of thermal cross-linking and self assembly

procedures

to

one-step

microwave-induced


polymerization

and


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self-assembly. With the functionalization of the abundant phenyl groups by chlorosulfonic acid, the obtained sulfonated ordered mesoporous phenolic resin nanosphere (SO3H-MPRN) exhibited the high catalytic reactivity and selectivity in the

aqueous

formaldehyde-anticipated

water-medium

Prins

reaction

and

water-medium Fisher-Indole reaction. The excellent catalytic efficiency could be attributed to the combined advantages of the nanosized short mesoporous channels and the hydrophobic pore surface, which can concentrate the reactants and also can decrease the reactant diffusion resistance and water competitive adsorption. Furthermore, it could be easily recycled and reused up to five times without significant loss of catalytic reactivity.

2. Experimental Section 2.1 Synthetic procedures Synthesis of phenol-formaldehyde resol oligomer/block copolymer composite: In a typical preparation, 0.80 g phenol and 2.1 mL 37% aqueous formaldehyde were mixed and allowed to stir at 40oC for 10 minutes with the stirring speed of 350 rpm. Then, 15 mL 0.10 mol/L aqueous sodium hydroxide solution was added into the mixture and the solution was heated to 70oC for another 0.50 h. After that, 0.96 g triblock copolymer Pluronic F127 (EO106PO70EO106) that dissolved in 15 ml H2O was poured in the solution. Then, the obtained mixture was stirred at 66°C for 2.0 h. Finally, 50 ml water was added to dilute the solution and allowed to stir for another 16 h, resulting in phenol-formaldehyde resol oligomer/block copolymer composite.


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Microwave-assisted preparation of ordered mesoporous phenolic resin nanosphere: A CEM microwave discover SP synthesizer was used for the synthesis of ordered mesoporous phenolic resin nanosphere. It provides microwave irradiation at 2450 MHz with a power of 300 W. In a typical microwave synthesis, 3.5 mL of the obtained composite solution and 11 mL H2O were added in the 35 mL reaction tube. The mixture was heated to 120°C and allowed to stir for 1.0 h. The yellow floccule was centrifuged and dried in vacuum. Then, the solid powder was calcinated at 380oC for 6.0 h in the N2 atmosphere to remove the F127 surfactant. The calcination process was as follows: kept 25oC for 10 min and then increased the temperature to 100oC at 1.0oC/min and maintained for 1.0 h, and finally raised the temperature to 380oC at 2.0oC/min. The final ordered mesoporous phenolic resin nanosphere was denoted as MPRN-1, MPRN-2, MPRN-3 and MPRN-4, by using 0.5 g, 0.6 g, 0.8 g and 1.0 g phenol in the initial synthesis of resol oligomers. Meanwhile, in the microwave heating process, the use of 100oC and 110oC instead of 120oC resulted in MPRN-5 and MPRN-6. Also, the use of 0.48 and 1.92 g F127 in the synthesis of phenol-formaldehyde resol oligomer/block copolymer composite gave the MPRN-7 and MPRN-8, respectively. Fabrication of sulfonated ordered mesoporous phenolic resin nanosphere: In a typical procedure, 1.0 g MPRN-3 sample was firstly dried in 110oC for 6.0 h and then introduced into 25 ml CH2Cl2 solution containing 10 ml chlorosulfonic acid. After degassing treatment, the reaction mixture was cooled to 0oC and allowed to stir for 12 h. The obtained solid product was filtrated, washed by ethanol and dried in vacuum at 80oC for 12 h, denoted as SO3H-MPRN.



2.2 Catalyst characterization The sulfur content was measured by elemental analysis on an Element Vario EL III analyzer. Fourier transform infrared (FTIR) spectra were obtained using Thermo Nicolet Magna 550 spectrometer. X-ray powder diffraction (XRD) data was acquired on a Rigaku D/maxr B diffractometer using Cu Kα radiation. Small-angle X-ray scattering (SAXS) patterns were recorded at a Bruker Nanostar U small-angle X-ray scattering system using CuKa radiation (40 kV, 35 mA). N2 adsorption-desorption isotherms were analyzed by Quantachrome NOVA 4000e analyzer. Specific surface areas (SBET) and average pore diameter (DP) were calculated by using BET and BJH models, respectively. Transmission electron microscopy images were observed by using a JEOL JEM-2011 transmission electron microscope. X-ray photoelectron spectroscopy experiments were performed on a Perkin-Elmer PHI 5000C ESCA system. All the binding energy values were calibrated by using C1S = 284.6 eV as a reference. Water and toluene vapour absorption measurements were carried out on an intelligent gravimetric analyse (Hiden Isochema IGA-002/3) by introducing a dosed amount of high-purity vapor directly into the sample chamber and recording the weight change after stable equilibrium pressure was reached.

2.3 Determination of acid capacity of SO3H-MPRN sample The acid capacity was determined by acid-base titration method. 0.15 g SO3H-MPRN sample was added into 50 mL 1.0 mol/L NaCl solution. The resulting


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suspension was stirred for 24 h at 25oC. Then, the solid sample was removed by filtration and the filtrate was divided into 20 mL and then titrated with 0.017 mol/L NaOH standard solution using phenolphthalein as an indicator. The end of the titration was reached when the color of filtrate turned from transparent to slightly pink, and kept this color for at least 30 s. A control experiment was performed with non-sulfonated ordered mesoporous phenolic resin nanosphere. The acid exchange capacity was calculated as follow: Dacid = CNaOH x (V1-V0)/mcatalyst, Where Dacid represents the acid capacity of the catalyst (mmol/g); CNaOH represents the concentration of the NaOH standard solution; V1 and V0 represent the volume of the NaOH standard solution consumed in the titration of sulfonated and non-sulfonated ordered mesoporous phenolic resin nanosphere, respectively; and mcatalyst represents the mass of catalyst.

2.4 Activity Test Reaction A: Aqueous formaldehyde-anticipated water-medium Prins reaction: In a typical run, 1.0 mmol α-methylstyrene, 3.0 mmol aqueous formaldehyde solution, a certain amount of SO3H-MPRN catalyst and 2.0 mL distilled water were mixed and allowed to react at 90oC for 2.0 h under mild stirring. The products were extracted with ethyl acetate, followed by analysis on a high performance liquid chromatography analyzer (HPLC, Agilent 6410 series Triple Quad) equipped with Agilent C18 column. The reaction conversion

was

calculated

based

on

α-methylstyrene


since

aqueous


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formaldehyde was in excess. In all the tests, the reproducibility was checked by repeating each result at least three times and was found to be within ±5%. Reaction

B:

Water-medium

Fisher-Indole

reaction:

0.50

mmol

benzoquinone, 0.50 mmol cyclohexanone, a certain amount of SO3H-MPRN-3 catalyst and 2.0 mL distilled water were mixed and allowed to react at 100oC for 2.0 h under mild stirring. The product analysis was performed according to the same procedure described in Reaction A. The reaction conversions were calculated based on benzoquinone. In order to determine the SO3H-MPRN catalyst recyclability, it was allowed to settle down after each run of Prins reactions and then the clear supernatant liquid was decanted slowly. The residual solid catalyst was reused with fresh charge of water, α-methylstyrene and aqueous formaldehyde for subsequent recycle under the same reaction conditions. Furthermore, the liquid phase of the reaction mixture was collected for element analysis after each reaction to test the sulfur leaching.

2.5 Adsorption testing To test the adsorption behavior of SO3H-MPRN catalyst, 50 mg catalyst was soaked in 50 mL water and oscillated at 25oC for 12 h, followed by adding 150 mL aqueous solution containing 300 ppm α-methylstyrene. The solution was sampled at given time intervals and the concentration of the left α-methylstyrene in solution was


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determined on a Shimadzu MC-2530 UV-vis spectrophotometer. The adsorption capacity was determined after reaching saturation adsorption.

3. Results and Discussion The synthetic process of ordered mesoporous phenolic resin-based solid acid (SO3H-MPRN) was shown in Scheme 1. Phenol and aqueous formaldehyde were used to prepare the phenolic resol oligomers under base-catalyzed polymerization. Then, the oligomers were self-assembled with triblock copolymer F127 via hydrogen-bonding interaction to generate the phenol-formaldehyde resol oligomer/block copolymer composite. Subsequently, these composites were treated under microwave heating to further polymerize and grow, resulting in the phenolic resin nanosphere. Afterwards, the template F127 was removed by calcination to produce the phenolic resin nanosphere with ordered mesoporous structure. Finally, SO3H-MPRN sample can easily obtained by the simple sulfonation treatment. Obviously, the properties of this mesoporous resin is mainly influenced by several factors including the resol oligomer properties, the surfactant amount, reaction temperature and time.22 Accordingly, these factors were investigated in the microwave-assisted preparation conditions. Shown in Figure 1a were nitrogen sorption isotherms of a series of mesoporous phenolic resin by changing the phenol amounts from 0.5 to 1.0 g. Each sample of this series was synthesized by the following process:



the initial synthesis of resol oligomers by using different amounts of phenol and 2.1 mL 37% aqueous formaldehyde, the as-prepared oligomers mixed with 0.96 g F127 surfactant, the obtained composite treated by microwave heating at 120oC for 1.0 h and the final template removal by calcination under nitrogen atomshpere. The results revealed that MPRN-1 and MPRN-3 samples were of type IV isotherm with H1 hysteresis loop, indicating the existence of mesoporous structure.23 Meanwhile, MPRN-3 sample showed a sharp capillary condensation, demonstrating that it had the uniform mesopores. But, MPRN-2 and MPRN-4 have the typical I isotherms. Also, MPRN-1 and MPRN-2 exhibited the low initial adsorption steps while MPRN-2 and MPRN-4 showed the irregular hysteresis loops. Meanwhile, the pore size distribution curves (Figure 1b) showed that MPRN-3 sample has the relatively uniformed pore sizes in comparsion with the other samples. There results confirmed that there were significant changes in the adsorbed amount of nitrogen and the capillary evaporation for the samples prepared using different phenol amounts. Furthermore, the textural parameters of MPRN samples including the specific surface area, average pore size and pore volume were summarized in Table 1. The specific surface area of MPRN-3 was 442 m2/g whereas MPRN-1, MPRN-2 and MPRN-4 had the surface areas of 139, 220 and 307 m2/g, respectively. Also, the average pore size and pore volume of MPRN-3 were higher than those of the other MPRN samples. Obviously, MPRN-3 has much better structural ordering than those of MPRN-1, MPRN-2 and MPRN-4. These


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findings were also consistent with X-ray powder diffraction analysis of these samples. Figure 1c revealed that all the MPRN samples exhibited one peak indicative of (100) reflection, suggesting that the existence of hexagonal arrayed pore structure.24 The decrease of the peak intensities of MPRN-1, MPRN-2 and MPRN-4 compared to MPRN-3 implied that the reduced mesoporous structure ordering. It could be explained that the amount of phenol is related to the molecular weight of the phenolic resin oligomer.25 The ordered degree of mesoporous structure is determined by hydrogen bonding driven self-assembly between the benzyl hydroxyl groups with the PEO blocks of F127 copolymer. Thus, the high ordered degree of MPRN-3 could be attibuted to the suitable molecular weight of the resin oligomer by using 0.80 g phenol in the initial preparation. Next, TEM images (Figure 2) of these samples revealed that MPRN-1 showed the irregular morphology with non-uniform inner pores. But, the periphral part of the sample was not porous structure, resulting in low surface area. For MPRN-2 sample, the porous structure in the whole particle was observed, but its morphology was still not regular. Interestingly, MPRN-3 had the uniform spherical morphology with around 500 nm size. Moreover, it displayed a two-dimensional hexagonal arrangement of one-dimensional channels with uniform size.26 However, the reduced ordered degree of mesopores was obtained in MPRN-4 sample, which was maybe due to the increased particle size that derived from the high molecular weight of the resin oligomers.



We also changed the microwave heating temperature in the prepartion of ordered mesoporous phenolic resin to obtain MPRN-5 and MPRN-6 by using 100oC and 110oC, respectively. As shown in Figure S1a-b, MPRN-5 exhibited the irregular morphology while MPRN-6 was spherical approximately, indicating that the temperature was related to the sample shape. Furthermore, the surfactant amount in the prepartion of resin oligomer/F127 composites was adjusted by using 0.48 g and 1.92 g F127, resulting in the formation of MPRN-7 and MPRN-8, respectively. TEM images (Figure S1c-d) revealed that these two samples displayed the spherical shape but the ordered degree of MPRN-7 and MPRN-8 was lower than that of MPRN-3, confirming that the surfactant amount determined the mesoporous structure, as suggested by the previously reported results by Zhao group.14 Then, the particle growth process was monitored by observing the morphology change of the resol oligomer/F127 composite in the different microwave heating time. The photograph (Figure 3) of the initial composite solution was clear with red colour. After microwave heating for only 5.0 minutes, the solution became turbid and the colour changed to brown. Then, the colour turned to light yellow after 15 minutes. The yellow and fluffy solid was obtained after 30 minutes and then it didn’t show the significant change in the longer time. SEM images (Figure 4) further revealed that the resol oligomer/F127 composite was the disordered micelle aggregate. After 5.0 minutes, the phase separation happened and then quickly transformed to irregular spheres. The increasing heating formed the individual round


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spheres after 1.0 h. But, the extended time up to 2.0 h caused the destruction of spherical morphology. These results demonstrated that the microwave heating can efficiently drive the resol oligomer/F127 composite to polymerize, separate and accumulate. The process is very fast because the composite is polar matter which can quickly absorb the microwave energy. On basis of these results, the optimized conditions were using 0.8 g phenol as the starting material in 120oC for 1.0 h under microwave radiation. To generate the acid speceis in the mesoporous phenolic resin, we transformed

the

phenyl

groups

in

the

framework

of

MPRN-3

to

benzenesulfonic acid groups by treating with chlorosulfuric acid, resulting in benzenesulfonic acid functionalzied phenolic resin nanosphere with ordered mesoporous

structure

(SO3H-MPRN).

We

optimized

the

amount

of

chlorosulfuric acid and sulfonation time and as expected the elemental analysis revealed that the sulfur content in the SO3H-MPRN sample was up to 2.15 mmol/g (Table S1). Also, the acid-base titration experiment determined that proton exchange capacity was 2.10 mmol/g, which suggested that the sulfur element nearly existed as sulfonic acid groups.27 It was evidenced by the FT-IR spectrum (Figure 5a)28, which revealed that the intense peak at 1040 cm-1 indicative of the symmetric stretch of the dissociated sulfonic groups was observed. Also, the little increase in at 1702 cm-1 that assigned to the bending mode of hydrated hydronium ions was obtained, which could be attributed the H3O+ ion chemically linked to the -SO3 groups in the SO3H-MPRN sample.



Furthermore, the S 2p XPS spectra (Figure 5b) showed that there was one peak at the binding energy of 168.4 eV, which corresponded to the sulfur species in the benzenesulfonic acid groups.29 Thus, we concluded that this in-situ transformation protocol could obtain the well-defined SO3H-functional groups in the framework of MPRN-3 sample. The small-angle X-ray scattering (SAXS) spectrum (Figure 6) for SO3H-MPRN exhibited one sharp peak and two peaks that can be indexed as the (100), (110), and (200) reflections, respectively, which was the typical 2D hexagonal mesostructure.30 For TEM measurement, the ground samples embedded in epoxy resin were cut in ultrathin sections and the result (Figure 7) clearly showed ordered mesopores and importantly all the pores were open to the outside of the particle, which is benefit to the mass transfer in the catalytic applications.31 Moreover, the surface area, pore size and pore volume of SO3H-MPRN were 410 m2/g, 3.5 nm and 0.40 cm3/g, which showed only a little decrease compared to the pristine MPRN-3 sample (Table 1). It could be explained that the existence of SO3H groups was in the resin framework rather than in the opening of the pore channels.13 The Prins reaction is an important carbon-carbon bond formation process that has the ability to obtain the common structural motif dioxanes.32 In the context, the use of aqueous formaldehyde as the starting material is believed as the most attractive protocol since it is cheap, easy to handle and less toxic.33 To investigate the utility of our SO3H-MPRN catalyst, water-medium Prins


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reaction with aqueous formaldehyde and α-methylstyrene was firstly carried out. The control experiment showed that the blank reaction without MPRN-3 sample did not generate any products (Table S2, entry 1). Then, we explored the catalyst amount in the presence of 1.0 mmol α-methylstyrene, 3.0 equiv. aqueous formaldehyde at 70oC for 2.0 h. The results showed that 5.0 mol% SO3H-MPRN could efficiently promote this reaction with 52.8% yield (Table S2). Next, we carefully investigated the reaction temperature and time as well as the amount of aqueous formaldehyde (Table 2). We found that the excellent conversion of 97.5% and selectivity of 100% were achieved with 5.0 mol% SO3H-MPRN and 4.0 equiv. aqueous formaldehyde at 90oC for 2.0 h. Interestingly, the decreased loading (2.50 mol%) of SO3H-MPRN catalyst also could obtain the good yield (92%) by simply extending the reaction time to 4.0 h. Moreover, the control catalysts including the commercial acidic resin Amberlyst-15, traditional mesoporous silica supported phenylsulfonic acid (SO3H-SBA-15) and sulfonated mesoporous phenolic resin with irregular morphology (SO3H-MPs) were tested. Obviously, Amberlyst-15 displayed quite low conversion and yield due to the low surface area and small pore size. Also, SO3H-SBA-15 only showed a little increase in the conversion and yield even with large surface area and ordered mesoporous structure (Figure S2), which was probably due to that the hydrophilic pore surface is detrimental to the catalytic performance. Furthermore, SO3H-MPs catalyst with irregular shape and long mesoporous channel was prepared (Figure S3). It exhibited the



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high conversion (89%) and yield (82%), but still inferior to SO3H-MPRN. Meanwhile, we calculated the TOF values of different solid acid catalysts and the results showed that SO3H-MPRN catalyst displayed the highest value in comparison with the control samples (Table 2). These data revealed the spherical nanostructure could efficiently decrease the diffusion limitation of the reactants in water, leading to the enhanced catalytic reactivity. To gain better insight into the hydrophobic effect, α-methylstyrene test (Figure 8a) in water over SO3H-MPRN and SO3H-SBA-15 were measured. The result showed that SO3H-MPRN can adsorb α-methylstyrene quickly with 55.2% adsorption precentage in 1.0 h while SO3H-SBA-15 only acheived with 25.6% adsorption precentage. Meanwhile, both toluene and water vapor absorption tests were employed to test the surface hydrophobicity of SO3H-MPRN. As shown in Figure 8b, all the isotherms were of type V, indicative of weak absorbent-absorbate

interaction.

However,

the

adsorption

capacity

of

SO3H-MPRN for toluene (46.1 wt.%) was much higher than that of water (19.3 wt.%). These results demonstrated that SO3H-MPRN catalyt is a hydrophobic solid acid, which could decrease the hydrophobic reactant diffusion limitation and meanwhile reduce water competitive adsorption.9 Next, we examined the scope of SO3H-MPRN catalyzed water-medium Prins reaction by using different aromatic olefins (Table 3). Similarly, p-chloro, p-bromo-, p-methyl substituted α-methylstyrene gave the excellent yields. Also, either the electron-accepting group (NO2-) or the electron-donating group (CH3O-) also


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led to high yields of the coupling products. Meanwhile, the use of styrene as the reactant delivered the product with 90.3% yield. Notably, 2-vinylnaphthalene with large molecular size also could be converted to product with excellent yield of 84.1%. These results confirmed its universal advantage in the aqueous formaldehyde participated Prins reaction. Furthermore, SO3H-MPRN catalyst was explored in the water-medium Fisher-Indole reaction, which is an extremely useful protocol to construct the important indole scaffold.34 Usually, this reaction is promoted by homogeneous acid catalysts in organic solvents, resulting in serious environmental hazards.35 Because of the high catalyst dosage and stoichiometric water byproduct, solid acids often displayed unsatisfied catalytic activity, especially in aqueous media.36 In this context, we firstly used cyclohexanone and phenylhydrazine as the probes and optimized the reaction conditions including catalyst amount, solvent usage and reaction temperature. Notably, SO3H-MPRN can obtain excellent conversion (93%) and good yield (85%) by using 20 mol% catalyst loading at 100oC for 2.0 h (Table 4). Interestingly, Amberlyst-15 displayed very low conversion (12%) and yield (10%), further confirming the advantage of SO3H-MPRN catalyst. Encouraged by the above result, the generality of the Fisher-Indole reaction was investigated. It showed the good catalytic reactivity for the reactions of phenylhydrazine with five or seven carbon cycloketones. Also, less activated phenylhydrazines with electron-withdrawing groups such as chlorine or nitro were also catalyzed to the desired products with the



moderate yields. Moreover, we compared the catalytic performances reported by the other groups in the water-medium Fisher-Indole reactions. Xu group synthesized a series of indoles through one-pot Fischer indole synthesis by using novel sulfonate ionic liquid in water.36 Furthermore, Li group successfully used the microwave heating to speed the reaction rate under the similar catalytic system.37 The major drawbacks in these reported results were the high acid amount (50 mol%) and the difficult recycle of their catalysts. To determine whether the heterogeneous acids or the leaching sulfonic acid was the real active species, the hot-filtration experiment was carried out.38 After reacting for 1.0 h that the conversion exceeded 50% in Prins reaction between α-methylstyrene and aqueous formaldehyde, the reaction mixture was directly separated by filtration to remove the SO3H-MPRN catalyst and then allowed the mother liquor to react for another 2.0 h under the same reaction conditions. We found than no signiﬁcant change in the α-methylstyrene conversion was observed, suggesting that the catalytic reactivity by the leaching sulfonic acid could be approximately excluded in the present system. Meanwhile, the mother liquid in the reaction mixture was collected after each reaction for elemental analysis. The result revealed that very low amount of sulfur element (less than 2.0 ppm) in the solution was detected. Thus, the present catalysis indeed was heterogeneous in nature rather than any dissolved acid species leached from the SO3H-MPRN catalyst.


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An attractive advantage of solid acid catalyst was its recycle and reuse, which reduce the work-up time and cost. Figure 9 showed the recyclability of the SO3H-MPRN catalyst during water-medium Prins reaction between α-methylstyrene and aqueous formaldehyde. No remarkable decrease could be found in the yields after being used repetitively five times. To investigate the influencing factors related to the slight decrease of the catalytic reactivity, we firstly employed the elemental analysis to test the leaching of the benzenesulfonic acid species in the reused SO3H-MPRN sample. The result revealed that the sulfur leaching was negligible because the sulfur content was 2.10 wt.% in the reused SO3H-MPRN catalyst and only about 2.3% of the sulfur species leached off after five repetitions. Low-angle XRD pattern (Figure S4) of the reused SO3H-MPRN catalyst revealed that ordered mesoporous structure was preserved. TEM image (Figure S5) further indicated that it could retain

the

spherical

morphology

and

mesoporous

structure.

N2

adsorption-desorption isotherm still displayed the typical IV isotherm with H1 hysteresis loop (Figure S6). Meanwhile, the inset pore size distribution curve revealed that it had the uniformed pores. Also, the surface area, pore size and pore volume of the reused SO3H-MPRN catalyst were 386 m2/g, 3.0 nm and 0.35 cm3/g, respectively, which were lower than those of the pristine SO3H-MPRN catalyst. These results showed that the slight decrease in the catalytic performances was mainly due to the partial destruction of the mesoporous structure of the reused SO3H-MPRN catalyst.39



4. Conclusions In summary, we have developed a time and energy efficient approach to prepare ordered mesoporous phenonic resin nanosphere by microwave heating resol oligomer/F127 template composite. This synthetic protocol overcame the traditional tedious preparation for the first time. Accordingly, benzenesulfonic acid functionalized mesoporous phenolic resin was obtained by sulfonating the phenyl groups in the resin framework. This solid acid catalyst can efficiently promote aqueous formaldehyde participated Prins reaction and Fisher-Indole reaction with water as the byproduct. This excellent catalytic performance could be attributed to the short mesoporous channels and hydrophobic pore surface, which reduced water adsorption on the acid active sites and decreased the mass transfer resistance in water. Moreover, due to the acid species that existed in the resin framework, the active site leaching was inhibited, leading to the increased stability in water. As a result, it can be reused for five times without the significant loss of catalytic reactivity. This work provides the novel synthetic approach towards the facile preparation of robust solid acid catalysts for more water-medium organic transformations.

Acknowledgments This work is supported by NSFC (51273112), Ministry of Education of China (PCSIRT_IRT_16R49), International Joint Laboratory on Resource Chemistry ACS Paragon Plus Environment

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(IJLRC), and the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning (TP2016034).

Supporting Information Available. The optimization of sulfonation conditions of MPRN, the investigation of the reaction conditions of SO3H-MPRN in Prins reaction and Fisher-Indole reaction, TEM images of MPRN and SO3H-SBA-15 samples, SEM image and TEM picture of SO3H-MPs sample, the XRD spectrum, N2 sorption isotherm and TEM image of the recycled SO3H-MPRN. This information is available free of charge via the Internet at http://pubs.acs.org/.

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32. Zhang, W. H.; Leng, Y.; Zhao, P. P.; Wang, J.; Zhu, D. R.; Huang, J. Heteropolyacid Salts of N-methyl-2-pyrrolidonium as Highly Efficient and Reusable Catalysts for Prins Reactions of Styrenes with Formalin. Green Chem. 2011, 13, 832-834. 33. Gu, Y. L.; Karam, A.; Jérôme, F.; Barrault, J. Selectivity Enhancement of Silica-Supported Sulfonic Acid Catalysts in Water by Coating of Ionic Liquid. Org. Lett. 2007, 9, 3145-3148. 34. Chen, H.; Eberlin, L. S.; Nefliu, M.; Augusti, R.; Cooks, R. G. Organic Reactions of Ionic Intermediates Promoted by Atmospheric-Pressure Thermal Activation. Angew. Chem. Int. Ed. 2008, 47, 3422-3425. 35. Liu, K. G.; Robichaud, A. J.; Lo, J. R.; Mattes, J. F.; Cai, Y. Rearrangement of 3,3-Disubstituted Indolenines and Synthesis of 2,3-Substituted Indoles. Org. Lett. 2006, 8, 5769-5771. 36. Xu, D. Q.; Wu, J.; Luo, S. P.; Zhang, J. X.; Wu, J. Y.; Du, X. H.; Xu, Z. Y. Fischer Indole Synthesis Catalyzed by Novel SO3H-Functionalized Ionic Liquids in water. Green Chem. 2009, 11, 1239-1246. 37. Li, B. L.; Xu, D. Q.; Zhong, A. G. Novel SO3H-Functionalized Ionic Liquids Catalyzed a Simple, Green and Efficient Procedure for Fischer Indole Synthesis in Water under Microwave Irradiation. J. Fluorine Chem. 2012, 144, 45-50. 38. Sheldon, R. A.; Wallau, M. I.; Arends, W. C. E.; Schuchardt, U. Heterogeneous Catalysts for Liquid-Phase Oxidations: Philosophers' Stones or Trojan Horses?. Acc. Chem. Res. 1998, 31, 485-493.


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39. Nakajima, K.; Tomita, I.; Hara, M.; Hayashi, S.; Domen, K.; Kondo, J. N. A Stable and Highly Active Hybrid Mesoporous Solid Acid Catalyst. Adv. Mater. 2005, 17, 1839-1842.



Sample SBET (m2/g) Dp (nm) MPRN-1 139 2.1 MPRN-2 220 3.0 MPRN-3 442 3.7 MPRN-4 307 3.8 SO3H-MPRN 410 3.5

Table 1. The textural parameters of MPRN and SO3H-MPRN samples.


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Vp (cm3/g) 0.18 0.23 0.41 0.36 0.40

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Table 2. Catalytic performances of different catalysts in water-medium aqueous formaldehyde participated Prins reaction.a O

O

+ aq. 2 H

Catalyst

S/C

SO3H-MPRN SO3H-MPRN SO3H-MPRN SO3H-MPRN SO3H-MPRN SO3H-MPRN SO3H-MPRN Amberlyst-15 SO3H-SBA-15 SO3H-MPs

20:1 20:1 20:1 20:1 20:1 20:1 40:1 40:1 40:1 40:1

Temperature (oC) 60 70 80 90 90 90 90 90 90 90

O

H

Formalin amount (equiv.) 3.0 3.0 3.0 3.0 2.0 4.0 3.0 3.0 3.0 3.0

a

Conversion (%) 42 60 88 97 71 97 99 40 44 89

Yield (%) 39 53 79 91 61 97 92b 23b 36b 82b

TOF (h-1) / / / / / / 11.8 4.76 5.24 10.6

Reaction conditions: 1.0 mmol α-methylstyrene, aqueous formaldehyde, 2.0 ml water, a certain amount of SO3H-MPRN catalyst, 2.0 h. b reaction time = 4.0 h



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Table 3. Catalytic performance of SO3H-MPRN catalyst in the water-medium Prins reaction between different aromatic olefins and aqueous formaldehyde.a

Entry

Reactant

Product O

97

89

95

88

99

93

99

90

93

84

Me O

O

4 MeO

O

O

5 O2N

O

O

6 O

O

7

a

91

O

3

O2N

99 Br

O

MeO

92

O

2

Me

99 Cl

O

Br

Yield (%)

O

1 Cl

Conversion (%)

Reaction conditions: 1.0 mmol aromatic olefin, 3.0 equiv. aqueous formaldehyde, 2.0 ml

water, 2.5 mol% SO3H-MPRN catalyst, 4.0 h.


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Table 4. Catalytic performances of SO3H-MPRN catalyst in the water-medium Fisher-Indole reactions.a Entry

ketone

hydrazine

Product

NHNH2

NHNH2

O

3

85

95

86

90

80

80

65

82

66

H N

NHNH2

H N

O

4

93

H N

O

2

Yield (%)

H N

O

1

Conversion (%)

Cl

NHNH2

Cl

O

O2 N

H N

NHNH2

5 O2N

a

Reaction conditions: 0.50 mmol ketone, 0.50 mmol hydrazine, 20 mol% SO3H-MPRN

catalyst, 100oC, 2.0 h.



Scheme 1. Illustration of the synthetic process of SO3H-MPRN sample.


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Figure 1. N2 adsorption-desorption isotherms (a), pore size distribution curves (b) and low-angle XRD patterns (c) of the MPRN samples.



Figure 2. TEM images of MPRN-1 (a), MPRN-2 (b), MPRN-3 (c) and MPRN-4 (d) samples.


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Figure 3. Photographs of the changes in the microwave-assisted preparation of MPRN-3 sample in the different times.



Figure 4. SEM images of the changes in the microwave-assisted preparation of MPRN-3 sample in the different times (a. 0 minute, b. 5 minute, c. 10 minute, d. 30 minute, e. 60 minute, f. 120 minute).


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Figure 5. FT-IR (a) and XPS (b) spectra of SO3H-MPRN sample.



Figure 6. Small-angle X-ray scattering (SAXS) spectra of MPRN-3 and SO3H-MPRN sample.


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Figure 7. Ultrathin sections of TEM images of SO3H-MPRN sample from [001] (a) and [110] (b) directions.



Figure 8. (a) α-methylstyrene adsorption profiles of SO3H-MPRN and SO3H-SBA-15 catalysts. (b) Toluene and water vapor adsorption tests of SO3H-MPRN catalyst.


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Figure 9. Recycling tests of the SO3H-MPRN catalyst in the Prins reaction between α-methylstyrene and aqueous formaldehyde. The reaction conditions are shown in Table 2.




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Microwave-Assisted Rapid Preparation of Mesoporous Phenolic Resin

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